The instant invention relates to a process and device to determine the diameter of a bobbin at a spinning station of a spinning machine. A fiber sliver of known thickness is fed at a determined speed to the spinning stations of the spinning machine, is spun there into a yarn and is then drawn off at a draw-off speed which is at a defined ratio to the sliver feeding speed. The yarn is then wound up on the bobbin at a winding speed synchronized thereto.
In order to determine the diameter of the bobbin, a known method consists in providing a stop that is variable in relation to the bobbin surface. The variable stop is a mechanical measure to determine the changing bobbin diameter at desired points in time. In this connection, continuous or discontinuous mechanical sensors which touch the bobbin surface are normally used.
As has been shown in practice, it is a condition for precise operation that constantly hard wound yarn bobbins be produced. With slightly wound bobbins the above-mentioned measuring device has the disadvantage that the signal to be formed for the bobbin diameter is distorted by the compression of the bobbin surface by the variable stop. This situation leads to lack of precision which reduces the probability of success in grasping a yarn end following a yarn breakage or else affects the precision of bobbin replacement. Mechanical scanning of the bobbin diameter by means of scanners (DE-OS 38 27 345) does not remove the enumerated disadvantages of the state of the art.
By using non-contact sensors, another method to determine the diameter of the yarn bobbin was found. An optical sensor continuously ascertains the changing diameter of the bobbin without having to come into contact with the bobbin surface. Optical scanning (DE-OPS 36 17 151, FIG. 3) has however the disadvantage that the optics on the textile machine can become dirty from dust, fibers and other particles, having as a consequence a distortion of the obtained signal.
A third possibility (U.S. Pat. No. 3,877,209) according to the state of the art consists in determining the rotational speed of the draw-off or winding roller, on basis of which the yarn length and thereby the diameter of the yarn body are determined.
This solution could not be used, in particular for the positioning of a yarn take-up device in relation to a bobbin surface, since influence factors found in practice distort the constant formation of gaps between the surface of the yarn body and the yarn take-up device and are not taken into consideration. These influence factors also lead to imprecision in finding the point in time at which bobbin replacement is to be initiated.
Such influence factors include:
possibly run-in pressure rollers of varying hardness, causing slip at the draw-off rollers; PA1 irregular wear of the slaving rubber on the friction rollers, causing variations in winding tension.
Because of the variety of materials capable of being spun as well as different production parameters (yarn thickness, winding tension) differences in yarn thickness and winding hardness are common, and these tend to render sensor-less determination of bobbin diameter as a guiding magnitude for control less precise than the utilization of sensors which actually measure the diameter.